Bisphenol-A (4,4′-dihydroxy-2,2-diphenylpropane or BPA) is produced by condensation of acetone with an excess of phenol in the presence of an acidic catalyst or a cation-exchange resin. The crude product, in addition to the desired bisphenol-A and unreacted phenol, contains unwanted by-products, such as bisphenol-A isomers, trisphenol and other higher molecular weight materials. The bisphenol-A is normally separated from the crude product by a single or a series of crystallization steps, leaving a mother liquor stream enriched in unwanted by-products, a portion of which stream is removed to purge unwanted by-products from the process. Alternately, the bisphenol-A may be separated from the crude product by a single or series of distillation steps, which also creates a stream enriched in unwanted by-products, a portion of which is removed. The removed stream may contain unreacted phenol and bisphenol-A as well as the unwanted by-products. Phenol is typically recovered from the removed stream by distillation, normally vacuum distillation, to create a residue stream. However, the viscosity of the residue stream increases as the weight fraction of the phenol in the residue stream decreases, making handling of this residue stream increasingly difficult.
Additionally, the bisphenol-A isomers, trisphenol and higher molecular weight materials in the residue stream may be subjected to thermal or catalytic cracking to generate phenol and isopropenylphenol (IPP) for enhanced recovery. The cracking step may be subsequent to or coincident with phenol recovery by distillation. However, the cracking process also increases the viscosity of the remaining residue stream, a tarry aromatic waste material containing less than 5 wt %, typically less than 1 wt %, phenol. Because this material is highly viscous, it is difficult to handle by conventional means and normally must be maintained at a temperature above 130° C., preferably above 160° C., to ensure its flowability.
There is therefore interest in developing methods of reducing the viscosity of BPA residue streams so as to facilitate their transportation, use and disposal. One such method is disclosed in U.S. Pat. No. 5,504,251, which teaches that the viscosity of bisphenol-A residual tars can be reduced by combination with the tar remaining when cumene is converted to phenol by the Hock process. The phenol tar is said to comprise 10-25 wt % phenol, 10-25wt % acetophenone, 3-5wt % dimethylbenzylalcohol, 20-40 wt % o,p-cumylphenol, and 5-10 wt % alpha-methylstyrene dimer, with the remainder being heavy tar. Although the weight ratio of bisphenol-A tar to phenol tar may range from about 99:1 to about 1:99, optimum reduction in viscosity of the heavy bisphenol-A tar is said to be obtained when the ratio of bisphenol-A tar to phenol tar is in the range of 1:10 to 1:1. The mixture of bisphenol-A tar and phenol tar can be thermally decomposed at a temperature of about 290° C. to about 360° C. to yield phenol, alpha-methylstyrene and cumene.
Other examples of cracking of mixtures of bisphenol-A tar and phenol tar can be found in, for example, U.S. Pat. Nos. 5,672,774 and 6,025,530.
According to the present invention, it has now been found that the viscosity of a BPA residue stream can reduced by combining the stream with (a) the waste heavy ends stream from an alkylaromatic production process, composed mainly of polyalkylated aromatic compounds, (b) additional phenol or a mixture of (a) and (b). In general, however, excessive use of phenol alone as the viscosity reducing diluent is undesirable since phenol is generally a higher value product than both the BPA residue stream and the alkylaromatic waste stream. Irrespective of the diluent employed, the blended stream is suitable for use as a boiler fuel.